The present invention relates to an arrangement for introducing a liquid medium, e.g. urea, into exhaust gases from a combustion engine
To meet prevailing exhaust cleaning requirements, today's motor vehicles are usually provided with a catalyst in the exhaust line to effect catalytic conversion of environmentally hazardous constituents of the exhaust gases to environmentally less hazardous substances. A method which has been employed for achieving effective catalytic conversion is based on injecting a reducing agent into the exhaust gases upstream of the catalyst. A reductive substance which forms part of, or is formed by, the reducing agent is carried by the exhaust gases into the catalyst and is adsorbed on active seats in the catalyst, resulting in accumulation of the reductive substance in the catalyst. The accumulated reductive substance may then react with and thereby convert an exhaust substance to a non-hazardous substance. Such a reduction catalyst may for example be of SCR (selective catalytic reduction) type. This type of catalyst is hereinafter called an SCR catalyst. An SCR catalyst reduces NOx in the exhaust gases. In the case of an SCR catalyst, a reducing agent in the form of urea solution is usually injected into the exhaust gases upstream of the catalyst. The injection of urea into the exhaust gases results in the formation of ammonia which then serves as the reductive substance which assists the catalytic conversion in the SCR catalyst. The ammonia accumulates in the catalyst by being adsorbed on active seats in the catalyst, and NOx present in the exhaust gases is converted to nitrogen gas and water when it is brought into contact in the catalyst with accumulated ammonia on the active seats in the catalyst.
When urea is used as reducing agent, it is injected into the exhaust line in the form of a liquid urea solution via an injection means. The injection means comprises a nozzle via which the urea solution is injected under pressure into the injection means in the form of a finely divided spray. In many operating states of a diesel engine the exhaust gases will be at a high enough temperature to be able to vaporise the urea solution so that ammonia is formed. It is difficult, however, to avoid part of the urea solution supplied coming into contact with and becoming attached to the internal wall surface of the exhaust line in an unvaporised state. The exhaust line, which is often in contact with and cooled by surrounding air, will be at a lower temperature than the exhaust gases within the exhaust line. When a combustion engine is run in a uniform way for a period of time, i.e. in steady-state operating conditions, no appreciable variations in the exhaust flow occur and the urea solution injected into the exhaust gases will therefore encounter substantially the same region of the exhaust line throughout said period of time. The relatively cool urea solution may cause local lowering of the temperature in that region of the exhaust line, which may lead to the formation in that region of a film of urea solution which is then entrained by the exhaust flow. When this film has moved a certain distance in the exhaust line, the water in the urea solution will boil away under the influence of the hot exhaust gases. Solid urea will remain and be slowly vaporised by the heat in the exhaust line. If the supply of solid urea is greater than the vaporisation, solid urea will accumulate in the exhaust line. If the resulting layer of urea becomes thick enough, the urea and its decomposition products will react with one another to form urea-based primitive polymers known as urea lumps. Such urea lumps may over time block an exhaust line.
It is therefore desirable that the injected urea solution be spread well out in the exhaust gases so that it is prevented from encountering substantially the same region of the exhaust line. A good spread of the urea solution in the exhaust gases also facilitates its vaporisation.
In a known arrangement from WO 2009/012885, an injection means is adapted to injecting a liquid medium into a space within a tubular casing, thereby bringing the injected medium into contact with exhaust gases which flow into said space via throughflow apertures which are distributed round the circumference of the casing. The mixture of exhaust gases and injected medium thus formed within the casing is led thence into a mixing duct. Exhaust gases are also caused to flow into the mixing duct via apertures distributed round the forward end of the casing in order thereby to create along the wall of the mixing duct an exhaust flow which hinders the injected medium from coming into contact with said wall.